Kofron et al U.S. Pat. No. 4,439,520 ushered in the current era of high performance silver halide photography. Kofron et al disclosed and demonstrated striking photographic advantages for chemically and spectrally sensitized tabular grain emulsions in which tabular grains having a diameter of at least 0.6 .mu.m and a thickness of less than 0.3 .mu.m exhibit an average aspect ratio of greater than 8 and account for greater than 50 percent of total grain projected area. In the numerous emulsions demonstrated one or more of these numerical parameters often far exceeded the stated requirements. Kofron et al recognized that the chemically and spectrally sensitized emulsions disclosed in one or more of their various forms would be useful in color photography and in black-and-white photography (including indirect radiography). Spectral sensitizations in all portions of the visible spectrum and at longer wavelengths were addressed as well as orthochromatic and panchromatic spectral sensitizations for black-and-white imaging applications. Kofron et al employed combinations of one or more spectral sensitizing dyes along with middle chalcogen (e.g., sulfur) and/or noble metal (e.g., gold) chemical sensitizations, although still other, conventional modifying compounds, such as metal compounds, were taught to be optionally present during grain precipitation.
An early, cross-referenced variation on the teachings of Kofron et al was provided by Maskasky U.S. Pat. No. 4,435,501, hereinafter referred to as Maskasky I. Maskasky I recognized that a site director, such as iodide ion, an aminoazaindene, or a selected spectral sensitizing dye, adsorbed to the surfaces of host tabular grains was capable of directing silver halide epitaxy to selected sites, typically the edges and/or corners, of the host grains. Depending upon the composition and site of the silver salt epitaxy, significant increases in speed were observed. Modifying compounds were taught to be optionally incorporated either in the host tabular grains or in the salt halide epitaxy.
In 1982 the first indirect radiographic and color photographic films incorporating the teachings of Kofron et al were introduced commercially. Now, 12 years later, there are clearly understood tabular grain emulsion preferences that are different, depending on the type of product being considered. Indirect radiography has found exceptionally thin tabular grain emulsions to be unattractive, since they produce silver images that have an objectionably warm (i.e., brownish black) image tone. In camera speed color photographic films exceptionally thin tabular grain emulsions usually have been found attractive, particularly when spectrally sensitized to wavelength regions in which native grain sensitivity is low--e.g., at wavelengths longer than about 430 nm. Comparable performance of exceptionally thin tabular grain emulsions containing one or more spectral sensitizing dyes having an absorption peak of less than 430 nm is theoretically possible. However, the art has usually relied on the native blue sensitivity of camera speed emulsions to boost their sensitivity, and this has retarded the transition to exceptionally thin tabular grain emulsions for producing blue exposure records. Grain volume reductions that result from reducing the thickness of tabular grains work against the use of the native blue sensitivity to provide increases in blue speed significantly greater than realized by employing blue absorbing spectral sensitizing dyes. Hence, thicker tabular grains or nontabular grains are a common choice for the blue recording emulsion layers of camera speed film.
Recently, Antoniades et al U.S. Pat. No. 5,250,403 disclosed tabular grain emulsions that represent what were, prior to the present invention, in many ways the best available emulsions for recording exposures in color photographic elements, particularly in the minus blue (red and/or green) portion of the spectrum. Antoniades et al disclosed tabular grain emulsions in which tabular grains having {111} major faces account for greater than 97 percent of total grain projected area. The tabular grains have an equivalent circular diameter (ECD) of at least 0.7 .mu.m and a mean thickness of less than 0.07 .mu.m. Tabular grain emulsions with mean thicknesses of less than 0.07 .mu.m are herein referred to as "ultrathin" tabular grain emulsions. They are suited for use in color photographic elements, particularly in minus blue recording emulsion layers, because of their efficient utilization of silver, attractive speed-granularity relationships, and high levels of image sharpness, both in the emulsion layer and in underlying emulsion layers.
A characteristic of ultrathin tabular grain emulsions that sets them apart from other tabular grain emulsions is that they do not exhibit reflection maxima within the visible spectrum, as is recognized to be characteristic of tabular grains having thicknesses in the 0.18 to 0.08 .mu.m range, as taught by Buhr et al, Research Disclosure, Vol. 253, Item 25330, May 1985. In multilayer photographic elements overlying emulsion layers with mean tabular grain thicknesses in the 0.18 to 0.08 .mu.m range require care in selection, since their reflection properties differ widely within the visible spectrum. The choice of ultrathin tabular grain emulsions in building multilayer photographic elements eliminates spectral reflectance dictated choices of different mean grain thicknesses in the various emulsion layers overlying other emulsion layers. Hence, the use of ultrathin tabular grain emulsions not only allows improvements in photographic performance, it also offers the advantage of simplifying the construction of multilayer photographic elements. As one alternative Antoniades et al contemplated the incorporation of ionic dopants in the ultrathin tabular grains as taught by Research Disclosure, Vol. 308, December 1989, Item 308119, Section I, Paragraph D.
Kuno U.S. Pat. No. 5,051,344 discloses silver iodobromide emulsions containing 0.1 to 4 mole percent iodide and, as grain dopants, 5.times.10.sup.-9 to 1.times.10.sup.-6 mole of an iridium compound and 5.times.10.sup.-9 to 1.times.10.sup.-6 mole of an iron compound per mole of silver. The grains are of a core-shell structure with the core containing a higher iodide content (at least 3 mole percent greater) than the shell. Kuno specifically prefers both the iridium and iron to be present in the shell. The object is to achieve high contrast with high-illuminance short-duration exposure, rapid processing, and better safe-light handling. The latter requirement, better safe-light handling, is a requirement of increased low intensity reciprocity failure. In other words, the emulsions are intended to be responsive to high intensity, short duration exposures, but relatively unresponsive to the low levels of illumination provided by safe-lights. Kuno recognizes that iridium reduces both high and low intensity reciprocity failure. Kuno's purpose in adding iron is to eliminate the effect of iridium in reducing low intensity reciprocity failure.
Daubendiek et al U.S. Pat. No. 5,494,789, (Daubendiek et al I) discloses photographic performance advantages in chemically and spectrally sensitized ultrathin tabular grain emulsions in which the chemical sensitization includes silver salt protrusions forming epitaxial junctions with the ultrathin tabular grains.
Daubendiek et al U.S. Ser. No. 297,430, (Daubendiek et al II) observes in addition to the photographic performance advantages of Daubendiek et al I improvements in speed-granularity relationships attributable to the combination of chemical sensitizations including silver salt epitaxy and iodide distributions in the host tabular grains profiled so that the higher iodide host grain concentrations occur adjacent the corners and edges of the tabular grains and preferentially receive the silver salt epitaxy.
Daubendiek et al U.S. Ser. No. 297,195, filed Aug. 26, 1994, commonly assigned and now allowed, titled ULTRATHIN TABULAR GRAIN EMULSIONS WITH SENSITIZATION ENHANCEMENTS, (Daubendiek et al III) observes additional photographic advantages, principally increases in speed and contrast, to be realized when the iodide concentration of the silver halide epitaxy on silver iodobromide ultrathin tabular grains is increased.
Olm et al U.S. Pat. No. 5,503,970, Aug. 26, 1994, commonly assigned, titled ULTRATHIN TABULAR GRAIN EMULSIONS WITH NOVEL DOPANT MANAGEMENT, (Olm et al I) observes additional photographic advantages to be realized when the silver salt epitaxy contains a dopant, such as an iridium or shallow electron trapping site providing dopant.
Olm et al U.S. Ser. No. 08/442,232, filed concurrently herewith and commonly assigned now allowed, discloses increased speed, reduced low intensity reciprocity failure, and low levels of pressure sensitivity when a tabular grain emulsion with higher iodide laminae located along opposed {111} major faces contains a divalent Group 8 (Fe.sup.+2, Ru.sup.+2 or Os.sup.+2) dopant and at least one ligand more electron withdrawing than fluoride ion at one location and a low intensity reciprocity failure reducing iridium dopant at another location with the grains.
Problem to be Solved
Notwithstanding the many advantages of tabular grain emulsions in general and the specific improvements represented by ultrathin tabular grain emulsions and color photographic elements, including those disclosed by Antoniades et al, there has remained an unsatisfied need for performance improvements in ultrathin tabular grain emulsions heretofore unavailable in the art as well as photographic elements containing these emulsions and for alternative choices for constructing emulsions and photographic elements of the highest attainable performance characteristics for color photography.
In addition there is a need in the art for ultrathin tabular grain emulsions that are "robust", where the term "robust" is employed to indicate the emulsion remains close to aim (i.e., planned) photographic characteristics despite inadvertent manufacturing variances. It is not uncommon to produce photographic emulsions that appear attractive in terms of their photographic properties when produced under laboratory conditions only to find that small, inadvertent variances in manufacturing procedures result in large quantities of emulsions that depart from aim characteristics to such an extent they cannot satisfy commercial requirements. There is in the art a need for high performance tabular grain emulsions that exhibit high levels of robustness or aim inertia, varying little from aim photographic characteristics from one manufacturing run to the next.
In attempting to modify the performance of ultrathin tabular grain emulsions through the inclusion of dopants to enhance photographic speed and reduce high intensity reciprocity failure (HIRF) it was observed that HIRF reductions attributable to the inclusion of iridium as a dopant were largely, if not entirely, lost when a shallow electron trapping site providing dopant, introduced to enhance photographic speed, was also present. Thus, an additional problem to be solved is obtaining both enhanced speed and reduced HIRF in an ultrathin tabular grain emulsion.